Synthetic Calcium Silicate Hydrate Research Articles

Effect of magnesium on calcium silicate hydrate (C-S-H)

The addition of MgCl2 to synthetic calcium silicate hydrate (C-S-H) with Ca/Si=0.8 lowers the pH values below 10. Experimental observations and thermodynamic calculations show the destabilization of C-S-H and the precipitation of magnesium silicate hydrate (M-S-H). In contrast, the addition of MgO to C-S-H increases the pH values up to 11.5 and the Ca/Si in C-S-H from 0.8 to ~1.0. At high pH, the formation of M-S-H and brucite is observed experimentally. Less brucite is present at 50 than at 20°C indicating a slow dissolution kinetic of brucite at 20°C. Thermodynamic modelling is in good agreement with the changes in C-S-H and M-S-H formation in the presence of MgO although no brucite is predicted.M-S-H is observed in the presence of both MgCl2 and MgO, is stable from pH7.5 to 11.5 and has a structure comparable to the structure of M-S-H prepared from MgO and SiO2 only.

Influence of the C-S-H amount on [Cl-]/[OH-] ratio of simulated concrete SPS and the corrosion susceptibility of steel

Two kinds of simulated concrete pore solutions (SPSs) were treated with different amounts of synthetic calcium silicate hydrate (C-S-H). The variation of the [Cl-]/[OH-] ratio in SPS was measured and the corrosion susceptibility of carbon steel in the SPS was investigated with potentiodynamic polarization, EIS and weight lose tests. The experimental results showed that for the SPS at pH 12.5, as the amount of C-S-H increases, the [Cl-]/[OH-] ratio increases thereby causing an increase in the corrosion susceptibility of the steel. While for the SPS at pH 9.7, with increasing C-S-H amount, the drop amplitudes of both [Cl-]/[OH-] ratio and steel corrosion rate first decrease and then increase, and a 3% C-S-H addition shows the best inhibition effect. XPS results demonstrate that after C-S-H treating in pH 12.5 SPS the [Fe3+]/[Fe2+] ratio in the film on steel surface is reduced while in pH 9.7 SPS the [Fe3+]/[Fe2+] ratio is increased. The different effects of the C-S-H amount on the two SPSs and the steel corrosion behavior result from the influences of C-S-H on the SPS pH, which is related to the composition of the SPS.

Characterization of the Bonds Developed between Calcium Silicate Hydrate and Polycarboxylate-Based Superplasticizers with Silyl Functionalities.

Major developments in concrete technology have been achieved with the use of polycarboxylate-based superplasticizers (PCEs) to improve the concrete rheology without increasing the mix water content. Currently, it is possible to control the fluidity of the fresh concrete and obtain stronger and more durable structures. Therefore, there is a strong incentive to understand the interactions between PCEs and cement hydrates at the atomic scale to design new customized functional PCEs according to the ever-increasing requirements of the concrete industry. Here, the bonding types generated between a PCE with silyl functionalities (PCE-Sil) and a synthetic calcium silicate hydrate (C-S-H) are analyzed using XRD, 29Si NMR spectroscopy, and synchrotron-based techniques, such as NEXAFS and EXAFS. The results indicated that the carboxylic groups present in PCE-Sil interact by a ligand-type bond with calcium, which modified not only the symmetry and coordination number of the calcium located at the surface of C-S-H but also the neighboring silicon atoms of the C-S-H. In addition, the silyl functionalities of the PCE-Sil generated covalent bonds through siloxane bridges between the silanol groups of PCE-Sil and the nonbonding oxygen located at the dimeric sites in C-S-H, forming new bridging silicon sites and subsequently increasing the silicate polymerization.

A luminescence line-narrowing spectroscopic study of the uranium(VI) interaction with cementitious materials and titanium dioxide.

Non-selective luminescence spectroscopy and luminescence line-narrowing spectroscopy were used to study the retention of UO2(2+) on titanium dioxide (TiO2), synthetic calcium silicate hydrate (C-S-H) phases and hardened cement paste (HCP). Non-selective luminescence spectra showed strong inhomogeneous line broadening resulting from a strongly disordered UO2(2+) bonding environment. This problem was largely overcome by using luminescence line-narrowing spectroscopy. This technique allowed unambiguous identification of three different types of UO2(2+) sorbed species on C-S-H phases and HCP. Comparison with spectra of UO2(2+) sorbed onto TiO2 further allowed these species to be assigned to a surface complex, an incorporated species and an uranate-like surface precipitate. This information provides the basis for mechanistic models describing the UO2(2+) sorption onto C-S-H phases and HCP and the assessment of the mobility of this radionuclide in a deep geological repository for low and intermediate level radioactive waste (L/ILW) as this kind of waste is often solidified with cement prior to storage.

Quantitative analysis of accelerated carbonation products of the synthetic calcium silicate hydrate(C–S–H) by QXRD and TG/MS

In this study, calcium silicate hydrate(C–S–H) with CaO/SiO2 ratio of 1.5 target mixture is synthesized and C1.27SH0.76 with 93.4 % purity is obtained. The dried C–S–H was compacted at 8 MPa to 20 mm × 20 mm × 20 mm for accelerated carbonation. The carbonation products and carbonation degree of synthetic calcium silicate hydrate are characterized and quantitatively evaluated by QXRD (Rietveld refinement), mass gain method, and TG/MS analysis. After accelerated carbonation at 0.2 MPa CO2 pressure for 2 h, the carbonation degree from mass gain method is 71.5 %, and from TG/MS analysis is 78.0 %. Calcite, aragonite, vaterite, and amorphous phase exist simultaneously in the accelerated carbonated samples, accounting for 33.98, 17.13, 18.74, and 30.15 %, respectively. Two mass-loss stages were observed for calcium carbonate decomposition. The first mass-loss stage (300–660 °C range) was mainly caused by the decomposition of aragonite and vaterite, and calcite formed in the accelerated carbonation process has a relatively high decomposition temperature which is mainly centered in the 660–800 °C range.

Structure and micro-nanomechanical characterization of synthetic calcium–silicate–hydrate with Poly(Vinyl Alcohol)

The principal phase of hardened Portland cement pastes is calcium silicate hydrate (C–S–H), which influences the physical and mechanical properties of construction materials. In this work calcium silicate hydrate (C–S–H) was synthesized, with the addition of Poly(Vinyl Alcohol) (PVA), for the development of C–S–H/polymer nanocomposites. Among the polymers available, PVA is indicated by the literature as one of the most viable for producing C–S–H/polymer complexes. However, no consensus exists regarding the kind of interaction. The resulting compounds were characterized by XRD, FT-IR, TGA, carbon content (CHN), TEM, SEM and elastic modulus and hardness were measured by instrumented indentation. The set of results presented do not confirm the intercalation of PVA in the interlayer space of C–S–H, but presented evidence of the potential for intercalation, since changes in the structure clearly occurred. A significant change in the micro-nanomechanical properties of C–S–H occurred in the presence of PVA.

Nano-mechanical characterization of synthetic calcium–silicate–hydrate (C–S–H) with varying CaO/SiO2 mixture ratios

Abstract Synthetic calcium silicate hydrate (C–S–H) made with calcium to silicate (C/S) mixture ratios of 0.9, 1.2 and 1.5 respectively is characterized. C–S–H was produced by extracting calcium oxide (CaO) from calcium carbonate (CaCO3) and then mixing it with micro-silica (SiO2) and deionized water to make slurry. The slurry was continuously mixed for 7 days, then the excess water was removed and thermo gravimetric analysis (TGA) was conducted. The drying method was equilibrated to 11% relative humidity (RH). The stoichiometric formula of the synthetic C–S–H were approximated as C0.7SH0.6, C1.0SH0.8 and C1.2SH2.4 for C/S mixture ratios of 0.9, 1.2 and 1.5 respectively. The dried powders were characterized using X-ray diffraction analysis (XRDA), and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The powders were also compacted with 95 MPa pressure and nanoindentation of the compacted specimens were then undergone to mechanically characterize the synthetic C–S–H. The experiments provide insight on the nanoscale mechanical characteristics of C–S–H.

Arsenic speciation in cemented paste backfills and synthetic calcium–silicate–hydrates

Arsenic (As) is a major pollutant in many mine tailings resulting from base metal and gold deposits. It can potentially cause environmental risks because of its high toxicity at low concentrations. A new tailings’ management technique has become popular in new mines, the cemented paste backfill (CPB). It consists of mixing tailings with water (typically 25%) and a low proportion of hydraulic binder (3–7%) to produce a paste that can be stored in underground mine openings. Even if CPB is mainly used for improving mining techniques and for tailings storage purposes, it could also provide environmental advantages by stabilizing contaminants such as As. In the present study, X-ray absorption fine structure (XAFS) investigations were conducted on CPB samples synthesized in the laboratory and spiked with As. XAFS analysis were also performed on a variety of As-bearing compounds (natural or synthetic), to provide a database of reference XAFS spectra. Among these reference samples, calcium–silicate–hydrates (C–S–H) spiked with various amounts of As through adsorption and co-precipitation were synthesized.The nature and amounts of the As species in the CPB samples, as well as As valence state were determined by linear combination of the reference spectra, in a least squares fitting procedure. As speciation in C–S–H was determined by extended X-ray absorption fine structure (EXAFS) fitting using theoretical curves from ab initio calculations.The results indicate that the binders promote the oxidation of As(III) to As(V) in the CPB. The As species formed in CPB are composed mainly of calcium arsenates, but other secondary minerals such as ferric arsenate can be present. Moreover, the use of fly ash binder could promote the formation of calcium–iron arsenates. However, CPB samples do not seem to contain arsenical C–S–H, as shown by the fitting results involving the synthetic C–S–H. In these samples, As reacts mainly through sorption mechanisms.

Disclosing the Mechanical Properties of Green Calcium-Silicate-Hydrates by Statistical Nanoindentation Techniques

In order to reduce CO2emissions, the cement industry has developed a new class cements. The Calcium-Silicate-Hydrates (CSH) that form are generally characterized by a low stoichiometric ratio for CaO and SiO2. This low C/S ratio affects the C-S-H layer structure and has a significant effect on the mechanical properties. This work exploits a novel statistical nanoindentation technique (SNT) to study the effect of the C/S ratio on the mechanical properties of synthetic CSH. Different CSH types were prepared by varying the C/S ratio of the starting materials. After undertaking a grid nanoindentation approach for each sample, the statistical analysis allowed extracting the mechanical properties, such as elastic modulus, hardness and creep. The results of this preliminary work shed new light on the implications of C-S-H stoichiometry on mechanical properties.

Nanostructure of Calcium Silicate Hydrates in Cements

Calcium silicate hydrate (CSH) is the major volume phase in the matrix of Portland cement concrete. Total x-ray scattering measurements with synchrotron x rays on synthetic CSH(I) shows nanocrystalline ordering with a particle diameter of 3.5(5) nm, similar to a size-broadened 1.1 nm tobermorite crystal structure. The CSH component in hydrated tricalcium silicate is found to be similar to CSH(I). Only a slight bend and additional disorder within the CaO sheets is required to explain its nanocrystalline structure.

Mechanical properties of calcium silicate hydrates

The dynamic mechanical properties of compacted samples of synthetic calcium silicate hydrate (C–S–H) were determined at variable stoichiometries (C/S ratio). The stiffness and damping properties of the C–S–H systems were monitored at various increments of mass loss from 11%RH following the removal of the adsorbed and interlayer water. The changes in the storage modulus (E′) and internal friction (tan δ) were discussed in terms of the state of water present in the nanostructure of C–S–H, the evolution of the silicate structure and the interaction of calcium ions in the interlayer region. Results were compared to those for the hydrated Portland cement paste and porous glass. It was shown that the C–S–H in the hydrated Portland cement has a complex yet analogous dynamic mechanical behavior to that of the synthetic C–S–H. The response of these systems upon the removal of water was explained by a layered model for the C–S–H. A mechanistic model was proposed to describe the changes occurring at various stages in the dynamic mechanical response of C–S–H.

Natural abundance high field 43Ca solid state NMR in cement science

This work is a systematic attempt to determine the possibilities and the limitations of the (43)Ca high field solid state NMR in the study of cement-based materials. The low natural abundance (0.135%) and small gyromagnetic ratio of (43)Ca present a serious challenge even in a high magnetic field. The NMR spectra of a number of cement compounds of known structure and composition are examined. The spectra of several phases important in cement science, e.g., anhydrous beta di-calcium silicate (beta-C(2)S) and tri-calcium (C(3)S) silicate were obtained for the first time and the relation of spectroscopic and structural parameters is discussed. The method was also applied to the hydrated C(3)S and synthetic calcium silicate hydrates (C-S-H) of different composition in order to understand the state of calcium and transformations in the structure during hydrolysis. The spectra of hydrated C(3)S reveals a calcium environment similar to that of the C-S-H samples and 11 A Tobermorite. These observations support the validity of using layered crystalline C-S-H systems as structural models for the C-S-H that forms in the hydration of Portland cement.

Stability of Synthetic Calcium Silicate Hydrate Gels in Presence of Alkalis, Aluminum, and Soluble Silica

A study is described of the stability (chemical composition and nano-structural change) of a synthetic calcium silicate hydrate gel in the presence of Ca(OH)2, the two main products of portland cement hydration, hours after alkalis, aluminum, and soluble silica were added to the mix. The sol–gel procedure was used to synthesize the gels. Calcium nitrate and sodium silicate solutions were used as the sources of calcium and silicon, respectively. The samples were prepared to a target Ca–Si ratio of 1.9. A 10-M NaOH solution was added to hold the pH at values greater than 13. All processes were conducted in a nitrogen atmosphere. The samples were characterized by using X-ray diffraction, Fourier transform infrared spectroscopy, nuclear magnetic resonance, transmission electron microscopy, and energy dispersive X-ray. The findings showed that addition of a high sodium hydroxide content led to silicate polymerization. The simultaneous addition of alkalis and aluminum induced a rise in the degree of silicate condensation in the gel [Q2(nAl) and possible cross-linking, Q3, via bridging tetrahedral aluminate], along with the formation of a calcium aluminosilicate hydrate similar to strätlingite. Finally, the simultaneous addition of alkalis, aluminum, and soluble silica favored the precipitation of an alkaline aluminosilicate hydrate gel with calcium replacing a high percentage of sodium and a composition clearly different from that of the other components.

Hydration of tricalcium silicate in the presence of synthetic calcium–silicate–hydrate

The early age-hydration of tricalcium silicate, the main chemical compound in Portland cement, was studied in the presence of synthetic calcium silicate hydrate (C–S–H) addition having C/S ratios = 0.8 and 1.2. Isothermal conduction calorimetry, scanning electron microscopy, differential scanning calorimetry and 29Si MAS NMR were employed in order to investigate events occurring during various stages of the hydration. The results that were analyzed using novel methods in cement chemistry showed that the addition of seeds of synthetic C–S–H significantly accelerated the hydration of C3S. The extent of the acceleration was dependant on the amount and chemical composition of the C–S–H seeds. It was suggested that the synthetic C–S–H significantly increased the rate and degree of dissolution of the C3S particles. It was also found that the nucleation and silicate polymerization of the C–S–H that formed during the hydration of the C3S phase was promoted. Direct evidence of the seeding effect was provided. The properties of the resulting C–S–H hydration products seemed to be dependant on the lime-to-silica ratio of the synthetic C–S–H. It was suggested that the silicate polymerization and chemical composition of the hydration products of silicate phases may be manipulated through C–S–H seeding. As the chemical and mechanical properties of C–S–H are largely controlled by its C/S ratio, this method should provide a unique tool for tailoring the nanostructure of the hydration products of Portland cement through the addition of selective C–S–H seeds for optimum engineering and durability performance.

Uranium(VI) Uptake by Synthetic Calcium Silicate Hydrates

AbstractThe immobilization of U(VI) by C-S-H phases under conditions relevant for the cementitious near field of a repository for radioactive waste has been investigated. C-S-H phases have been synthesized using two different procedures: the “direct reaction” method and the “solution reaction” method.The stabilities of alkaline solutions of U(VI) (presence of precipitates or colloidal material) were studied prior to sorption and co-precipitation tests in order to determine the experimental U(VI) solubility limits. These U(VI) solubility limits were compared with the U(VI) solubilities obtained from thermodynamic speciation calculations assuming the presence of combinations of different solid U(VI) phases. The solid phase controlling U(VI) solubility in the present experiments was found to be CaUO4(s).The U(VI) uptake kinetics and sorption isotherms on C-S-H phases with different C:S ratios were determined under various chemical conditions; e.g., sorption and co-precipitation experiments and different pH’s. U(VI) was found to sorb fast and very strongly on C-S-H phases with distribution ratios (Rd values) ranging in value between 103 L kg-1 and 106 L kg-1. Both sorption and co-precipitation experiments resulted in Rd values which were very similar, thus indicating that no additional sorption sites for U(VI) were generated in the co-precipitation process. Furthermore, C-S-H synthesis procedures did not have a significant influence on U(VI) uptake. The U(VI) sorption isotherms were found to be non-linear, and further, increasing Ca concentrations resulted in increasing U(VI) uptake. The latter observation suggests that U(VI) uptake is controlled by a solubility-limiting process, while the former observation further indicates that pure Ca-uranate is not the solubility-limiting phase. It is proposed that a solid solution containing Ca and could control U(VI) uptake by C-S-H phases.

Synthetic Layered Silicate as a Carrier for Liquid Ingredients for the Rubber‐ and Tire Industry

AbstractWe propose the use of novel materials based on synthetic calcium silicate hydrate (CSH) for the rubber‐ and tire industry. We found that the synthetic silicate belongs to the family of layered calcium silicate hydrates. Due to its layered structure and inert surface it easily adsorbs liquid components of rubber compounds, such as bis(triethoxysilylpropyl) tetrasulfide (TESPT) and liquid polysulfide (LP). Then, in processing, the CSH can smoothly release these components. The water, trapped in the material's gallery layers, plays the role of a catalyst in the silane coupling on silica's surface. In addition, we used this silicate as a component in promoter systems in order to improve adhesion stability between a brass‐coated steel cord and rubber. The application of the calcium silicate hydrate as an inert substrate for the promoter system allowed the development of materials that have comparable adhesion strength with most commercial promoters.

Non-ideal solid solution aqueous solution modeling of synthetic calcium silicate hydrate

New data relevant to calcium silicate hydrate (C–S–H) gels prepared at room temperature have been obtained over a time period of up to 112 weeks. X-ray diffraction (XRD) indicates equilibrium was attained after 64 weeks. Coupled with fourier transform infrared (FT-IR) spectroscopy, a phase change in C–S–H gel at Ca/Si ≈ 1.0 was identified and the occurrence of portlandite as a distinct phase for Ca/Si > 1.64. The incongruent dissolution of C–S–H gel was modeled as a non-ideal solid solution aqueous solution (SSAS) between the end-member components CaH 2SiO 4 (CSH) and Ca(OH) 2 (CH) using equations defining the solidus and solutus curves on a Lippmann phase diagram. Despite being semi-empirical, the model provides a reasonable and consistent fit to the solubility data and can therefore be used to describe the incongruent dissolution of C–S–H gels with compositions Ca/Si ≥ 1.0.

29Si chemical shift anisotropies in calcium silicates from high-field 29Si MAS NMR spectroscopy.

29Si chemical shift anisotropy (CSA) data have been determined from (29)Si MAS NMR spectra recorded at 14.1 T for a number of synthetic calcium silicates and calcium silicate hydrates. These are beta- and gamma-Ca(2)SiO(4), Ca(3)SiO(4)Cl(2), alpha-dicalcium silicate hydrate (alpha-Ca(2)(SiO(3)OH)OH), rankinite (Ca(3)Si(2)O(7)), cuspidine (Ca(4)Si(2)O(7)F(2)), wollastonite (beta-Ca(3)Si(3)O(9)), pseudowollastonite (alpha-Ca(3)Si(3)O(9)), scawtite (Ca(7)(Si(6)O(18))CO(3).2H(2)O), hillebrandite (Ca(2)SiO(3)(OH)(2)), and xonotlite (Ca(6)Si(6)O(17)(OH)(2)). The (29)Si MAS NMR spectra of rankinite and wollastonite clearly resolve manifolds of spinning sidebands from two and three Si sites, respectively, allowing the CSA parameters to be obtained with high precision for each site. For the (29)Si Q(1) sites in rankinite and cuspidine, the CSA asymmetry parameters (eta(sigma) approximately 0.6) contrast the general expectation that sorosilicates should possess small eta(sigma) values as a result of the nearly axially symmetric environments of the SiO(4) tetrahedra. The (29)Si CSA parameters provide an improved insight into the electronic and geometric environments for the SiO(4) tetrahedra as compared to the values solely for the isotropic chemical shift. It is shown that the shift anisotropy (delta(sigma)) and the CSA asymmetry parameter (eta(sigma)) allow a clear distinction of the different types of condensation of SiO(4) tetrahedra in calcium silicates. This relationship may in general be valid for neso-, soro-, and inosilicates. The CSA data determined in this work may form a valuable basis for (29)Si MAS NMR studies of the structures for tobermorites and calcium silicate hydrate phases resulting from hydration of Portland cements.

Testing of models of the dissolution of cements—leaching of synthetic CSH gels

The exhaustive leaching of a range of synthetic calcium silicate hydrate (CSH) gels in demineralised water has been studied. This has tested the applicability of experimental data and models based on the static dissolution of synthetic CSH gels in pure water (dissolution of a set of CSH gels with a range of compositions, each at effectively constant composition) to the description of the slow leaching of cementitious materials by flowing waters. This provides additional confidence in the modelling of the long-term evolution of conditions in a repository backfilled with cementitious materials and thereby helps underpin the assessment of the performance of such a repository. The results demonstrate that the initial stages of the dissolution of a range of CSH gels in demineralised water are apparently incongruent, followed by an approach to congruent behaviour. This behaviour is broadly similar to that predicted from the static dissolution of CSH gels. The congruently dissolving composition is found at a Ca/Si ratio of approximately 0.8 to 0.9. This is comparable to the ratio of about 0.8 suggested by static dissolution. However, sequential leaching of CSH gels apparently results in a lower solubility for the congruently dissolving composition than is predicted from static dissolution. The experimental results are compared with data obtained from static dissolution measurements for synthetic CSH gels and with the predictions of a computer model based on the Berner model of cement dissolution. The need to revise the model of dissolution to take account of the behaviour of CSH gels during sequential leaching is examined.

Sr 2+ uptake and leachability study on cured aluminum-substituted tobermorite–OPC admixtures

Synthetic calcium silicate hydrate (C-S-H), which is similar to 1.1 nm tobermorite mineral, has demonstrated capacity to selectively pick up Sr 2+ cations from mixed cationic solutions in the presence of 1000 times concentration of Na +, K +, Ca 2+ and Ba 2+ ions. The selective uptake of Sr 2+ from mixed cationic solution has been quantified as distribution coefficient ( K d) and decontamination factor ( D f). The mass balance data have been generated from the solution analysis for Ca 2+⇆Sr 2+ system after equilibration. The data show that ratio of the amount of Sr 2+ incorporated in the matrix to the amount of Ca 2+ released from the tobermorite is approximately equal to 1:1. The position of Al and Si in the aluminum-substituted tobermorite has been characterized by 27Al and 29Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Because 1.1 nm tobermorite is a powder, experiments were carried out on monolithic samples solidified with various amounts of ordinary portland cement (OPC). Leach rate studies were performed to study the retention power of Sr 2+ in these monoliths. The leach rate of Sr 2+ decreases drastically due to the addition of 10–40% of tobermorite to the OPC. The Sr 2+ retention capacity is expressed in terms of meq/100 g of exchanger. The leach rate studies show that Sr 2+ retention by various tobermorite containing OPC blocks increases with increasing weight percentage of this 1.1-nm Al-substituted tobermorite, which may be used as an additive to make the waste form.